This invention relates generally to optical communications. More particularly, it relates to semiconductor optical amplifiers and other SOA related devices.
As the demand for bandwidth in optical transmission increases, intense development efforts are focused on new amplification methods. There is also a great demand for economical amplification methods in low cost markets such as metro and enterprise networks. Semiconductor optical amplifiers may be used to provide optical amplification in a device that is less expensive, more compact, and more reliable than other commonly used amplifiers such as rare earth doped fiber amplifiers. Furthermore, semiconductor optical amplifiers can provide gain over a much broader range of wavelengths than other types of optical amplifiers. Due to its compact size and integrability the semiconductor optical amplifier can also be used in many other applications such as wavelength conversion, light modulation, gain spectra tilt control, dynamic gain equalization, etc.
Previous SOA designs utilize a single-pass design as depicted in FIG. 1. As shown in FIG. 1, an SOA 100 comprises an active layer 102 disposed between lower and upper cladding layers 104, 106. A waveguide structure 108 is incorporated into the SOA 100 to confine optical signals to an active region centered on the active layer 102. The SOA 100 receives radiation in the form of an input optical signal OSI from an input fiber 112. A first lens system 114 couples the input optical signal OSi from the input fiber 112 to the SOA 100. The SOA amplifies the input optical signal OSi and produces an amplified output signal OSo. A second lens system 116 couples the output optical signal OSo to an output fiber 118.
The single-pass amplifier design suffers from certain drawbacks. One drawback of the single-pass SOA design is that very precise alignment is often necessary for the two lenses 114, 116 and the two fibers 112, 118 in order to properly couple radiation into and out of the SOA 100. The precise alignment in a single-pass SOA design increases the number of components, which adds to the complexity and cost of the SOA 100.
Another drawback is that a single-pass amplifier suffers from certain thermal management problems. A SOA, such as SOA 100 tends to generate heat as a byproduct of the amplification process. This heat tends to raise the temperature of the SOA 100. Changes in temperature can effect optical properties, such as the index of refraction and mechanical properties, such as the effective length and width, of the SOA 100. Moreover, changes in temperature tend to deteriorate the performance and reliability of the SOA 100. To overcome this, the SOA 100 is usually attached to a heat sink 120 as shown in the side elevation of FIG. 1B. The heat generated within the SOA 100 flows into the heat sink, usually by thermal conduction. To enhance thermal stability of the SOA 100, a large heat sink 120 is generally preferred. Unfortunately, the lenses 114, 116 of the single-pass SOA 100 occupy a considerable amount of space, which limits the space available for the heat sink 120. Furthermore, isolators 113, 115 are often required to prevent signals from undesirably coupling from the amplifier 100 to the input fiber 112 or from the output fiber 118 to the amplifier 100. The isolators 113, 115 add to the complexity and cost of apparatus that use the amplifier 100.
FIG. 1C depicts an amplifier 100C that uses a xe2x80x9cVxe2x80x9d shaped waveguide 108C and a reflector 115. The reflector 115 is located proximate the point of the xe2x80x9cVxe2x80x9d. Incoming signals travel along one leg of the V, are reflected by the reflector and travel out of the amplifier along the other leg of the V. Although this arrangement allows the use of a large portion of the amplifier, it is still a single-pass design and suffers from the complexity and cost problems associated with other single-pass designs. Furthermore, the angle between the legs of the v-shaped waveguide 108C complicates the alignment of optical fibers that carry signals into and out of the amplifier 100C.
An additional drawback of single-pass amplifiers, such as SOA 100, is that two different channels can deplete the gain of the SOA in the same location. Gain depletion due to a channel at one wavelength can effect the gain for a channel at another wavelength resulting in undesirable cross talk between the two channels.
Furthermore, single-pass SOA""s tend to have low depletion at the input end. FIG. 1D schematically illustrates the gain depletion for the SOA 100 as a function of position.
Radiation is incident on SOA 100 at an input end 103, e.g. proximate lens 114. Where the gain depletion is low the amplification tends to be high and vice versa. In the SOA 100, the gain depletion increases and the amplification decreases towards an output end 105, e.g. proximate lens 116. Spontaneous emission, i.e. noise, generated near the input end 103 tends to get amplified more than spontaneous emission generated near the output end 105. The amplified spontaneous emission results in an undesirably noisy output signal.
There is a need, therefore, for an improved semiconductor optical amplifier that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide an optical amplifier design that is easier to align than uses fewer components than a single-pass design. It is an additional object of the invention to provide an optical amplifier that is less subject to cross-talk than a single-pass design. It is a further object of the invention to provide an optical amplifier with a more uniform gain depletion distribution than a single-pass design. It is an additional object of the invention to provide an optical amplifier that can accommodate a larger heat sink than a single-pass design. It is an additional object of the invention to provide an optical amplifier having low cost and high reliability.
The above objects and advantages are attained by a novel optical amplifier. The optical amplifier generally comprises an amplification section and a reflector. The amplification section receives optical signals at a first end face. The reflector, which is optically coupled to the second end face reflects optical signals back toward the first end face along substantially the same path through the amplifier. The amplification section may include various amplifier types including a semiconductor optical amplifiers (SOA), rare-earth-doped amplifiers such as erbium doped amplifiers, rare earth doped waveguide amplifiers, such as erbium doped waveguide amplifiers on silicon, glass or polymer substrates, parametric amplifiers or polymer amplifiers. In a particular embodiment, the amplifier is an SOA having an active layer sandwiched between two cladding layers. The reflector reflects at least a portion of the input optical signal at the first end face of the optical amplifier back toward the second. The amplification section of the amplifier interacts with the input and reflected optical signals to produce an amplified optical signal. A distance between the first and second end faces may be chosen such that standing wave intensity patterns for optical signals of different wavelengths have one or more peaks at substantially different locations in the amplifier. The reflector may optionally be characterized by a wavelength-dependent reflection coefficient to facilitate adding or removing optical signals of selected wavelengths from the amplifier.
Embodiments of the present invention provide optical amplifiers for communications apparatus that use fewer components, with less cross-talk and more uniform gain depletion distribution than single-pass design.
An alternative embodiment of the invention provides a waveguide amplifier having a bent waveguide to provide for parallel alignment of input and output fibers. The waveguide amplifier includes an amplification section having first and second end faces, wherein the first end face includes an anti-reflection coating. First and second optical fibers may be coupled to the first end face. A reflector optically is coupled to the second end face. The amplifier further includes a waveguide structure for guiding optical signals within the amplification section. The waveguide structure has a modified xe2x80x9cVxe2x80x9d shape with a first leg and a second leg. The first and second legs intersect at the second end face. One of the two legs is bent so that both the first and second legs intersect the first end face parallel to each other. This structure allows easier alignment of optical fibers for coupling optical signals into and out of the amplification section than in prior single-pass designs.